It is known to heat steel components to the so-called austenizing temperature and to harden them thereafter by quenching. For heating to the austenizing temperature, so-called quenching furnaces are known, into which the components are placed and appropriately heated and thereafter removed.
Since the beginning of the nineties, not only machine components made of steel, such as shafts and bearings, for example, are being hardened, but also car body components. This process is also called press-hardening steel (PHS) process. In connection with this technology, a steel plate is heated to the austenizing temperature for obtaining car body parts with a high degree of strength, and is subsequently re-shaped and simultaneously rapidly cooled, so that the known hardening effect occurs. The strength of the car body material is increased by these hardening processes to as much as 1,500 MPa, for example. By means of this greatest possible strength of the material, it has become possible to clearly increase the safety of modern vehicles in case of accidents, while maintaining the same weight of the car body.
Up to now, continuous furnaces, but also roll-over-type furnaces, in which the panels or pre-shaped parts were heated, have been used for heating such steel plates. Because considerable oxidation (scaling) already occurs at the surface of these components at these temperatures, such hardening or heating furnaces are customarily operated with the use of a protective gas.
It is furthermore known to design panels or pre-shaped components with a coating of aluminum, or of an alloy consisting to approximately one-half of aluminum or zinc. With such coatings it is possible under certain circumstances to omit the protective gas atmosphere.
At present, continuous furnaces, such as roll-over-type furnaces, but also rotary tubular furnaces, in which the components remain for extended periods of time, are used for heating car body parts. The car body parts are thereafter transported to presses and are made into the desired shape there.
The existing furnaces have the disadvantage that the transport system is arranged in the interior of the furnace and is therefore highly prone to damage. Maintenance of the transport system can only take place when the furnace has cooled off. Added to this is that the positions of the car body parts are not fixed and that positional displacement of the components occurs in the course of the transport through the furnace, so that the components must first be repositioned when leaving the furnace, so that they can be removed thereafter and transported to the press. In this connection it is a disadvantage that the components which are not correctly and properly positioned, rapidly cool during the repositioning. To compensate these heat losses, the components are heated from the start in the furnace to temperatures which lie clearly above those which would be required for press hardening. The temperature required for press hardening lies customarily at 930° C.
Because all components are heated to a higher than necessary temperature, but only a part of the components needs to be repositioned, components reach the re-shaping tools at different temperatures. However, different temperatures also mean that the obtained hardnesses are not uniform and fluctuations exist here. This also means that the components of different initial temperatures possibly also have different end temperatures, so that deformations can also occur.
It is moreover disadvantageous in connection with customary furnaces that product carriers weighing more than 60 kg are employed. Following the heating of the car body parts, these carriers are run out of the furnace and are transported back to the entrance outside of the furnace, where then a new component can be placed on these carriers. In the course of being moved out, back and in again, the carrier loses up to 200° C. This heat loss must be compensated again in the furnace, i.e. the furnace must not only heat the car body components, but also the carriers in addition, which requires additional energy.
A further disadvantage in connection with known roller hearth furnaces is that roller hearth furnaces are limited in their width. Since the rollers are made of a ceramic material or heat-resistant steel, bending because of the influence of heat occurs at too large a width of the furnace, which cannot be tolerated in the present case. This furthermore leads to damages of the rollers because of shifting loads.
It is the object of the invention to create a method by means of which steel components, and in particular sheet steel components which are to be subjected to press-hardening, can be efficiently and cost-effectively heated, production quality is balanced, and energy is saved.
It is a further object to create a device for executing the method.